1002 ‫אוקטובר‬
‫טקס חלוקת מלגות ע"ש אורן ברקו ז"ל‬
The Vertical Distribution of Dissolved Inorganic N-Species and the
Phytoplankton Community Structure in the
Gulf of Aqaba
Efrat David, Boaz Lazar and Anton Post
“H. Steinitz” Marine Biology Laboratory, Interuniversity Institute for Marine Science,
Coral Beach, POB 469, Eilat 88103, Israel
Institute of Earth Sciences, The Hebrew University of Jerusalem, Edmond Safra
Campus, Jerusalem 91904, Israel
Dissolved inorganic nitrogen (DIN) is the major nutrient limiting the productivity in the
oceans. In most aquatic environments, N in DIN appears in three oxidation states,
ammonium+ammonia (-3), nitrite (+3) and nitrate (+5). Other species such as N2 and
N2O are important only in special oceanic environment (areas of nitrogen fixation,
anoxic environments, etc.). Ammonium, a prime metabolic product, is produced by
heterotrophic degradation of organic nitrogenous compounds. Ammonium is the
preferred nitrogen source in all phytplankton species and is readily assimilated by
bacteria because its N is in the same oxidation state as proteins. Due to its
availability, ammonium rarely accumulates in normal marine environments. Its portion
that is regenerated within the photic zone supports the large “regenerated”
production. Nitrate, although almost lacking from surface waters, is the most
abundant DIN species in the sea. It accumulates in deep water below the photic zone
as the result of nitrification in the absence of phytoplankton assimilation. The large
nitrate reservoir supports the so-called “new” oceanic productivity by upwelling or
vertical mixing of deep water into the photic zone, supplying phytoplankton with “new”
deep-water DIN. Nitrite is an intermediate DIN species. Its water column vertical
distribution results from nitrification, excretion, assimilation and vertical transport.
Nitrate and nitrite vertical distributions in the Gulf of Aqaba change drastically
between winter and summer as a result of vertical mixing and stratification,
respectively. In order to follow their concentrations we used the standard automated
FIA method. The ammonia was measured manually on board using a modified new,
sensitive method that yielded the first reliable ammonia measurements in the Gulf.
The low ambient concentrations of ammonium demanded a strict implementation of
contamination-free sampling procedures and the use standard additions to each
sample to correct for varying sensitivity and matrix effect.
During summer, the photic zone down to the deep chlorophyll maximum (DCM) was
characterized by very low concentrations of nitrate, nitrite and ammonium. The
ammonium and nitrite show two distinct narrow (ca. 20 m thick) maxima below the
DCM. The ammonium peak is located ca. 15 m above the nitrite peak. The nitrite
peak vanishes at the base of the photic zone where the nitrate concentration rises
sharply. While the nitrite peak exist during the whole stratified season, the
ammonium peak was observed only several times. The deep water (below the photic
zone) nitrate inventory increases gradually during all stratified summer season.
During winter the mixed layer shows constant concentrations of all measured DIN
species. Concentration of nitrate within the mixed layer is usually higher than the
concentration within the photic zone during summer. The concentration of nitrite
during mixing is high, reaching sometimes the levels of nitrate. Ammonium
concentrations remained very low showing no distinct maximum. The onset of
summer stratification was accompanied by the development of ammonium peak in
surface waters.
Water column phytoplankton community structure followed the vertical distribution of
DIN species. During deep winter mixing when nitrate concentration within the photic
zone is high, eukaryotic species dominated the phytoplankton community. In
contrast, eukaryotic species population was minimal throughout the summer stratified
period, hence, they may thus thrive best under nitrate replete conditions. In the
spring, during the onset of stratification, nitrate was progressively depleted from the
surface layers and ammonium levels increased, a bloom of the unicellular
cyanobacterium Synechococcus was apparent. Synechococcus cultures have been
shown to utilize nitrate and ammonium equally well. The Synechococcus bloom was
replaced by Prochlorococcus populations which dominated the photic zone down to
150 m and deeper. Dimly fluorescent cells of Prochlorococcus were found in the
upper 60-80 m, characteristic for the upper layer of the photic zone in summer when
nitrate and nitrite are absence. Prochlorococcus was long believed to be incapable of
nitrate and nitrite utilization. However, a second population of brightly fluorescent
Prochlorococcus cells, apparently low light adapted ecotypes were observed at the
primary nitrite maximum (PNM) and below. Numerically this population formed the
only significant phytoplankton group at depth. Abundances of the bright fluorescent
Prochlorococcus cells correlated well with the PNM and lend a basis to challenge the
paradigm that Prochlorococcus is incapable of nitrate and nitrite utilization. Indeed
physiological experiments have shown that some Prochlorococcus may utilize nitrite
and recently a nitrate utilizing Prochlorococcus type was brought into culture and
awaits further characterization. Experiments in which surface waters were mixed with
high nitrate deep water support the findings above that eukaryotic species thrive on
nitrate and are the main “new” producers whereas prokaryotic species dominates the
“regenerated” production on ammonium (and urea?) within the photic zone.
Our findings indicate that the bottom half of the photic zone of the Gulf of
Aqaba in summer forms an intricate system in which light levels,
ammonium availability, differential depth distributions of ammonium
oxidizing bacteria and of Prochlorococcus ecotypes capable of
nitrate/nitrite assimilation together shape the ammonium and nitrite
maximum as well as the nitracline. Present studies aim to characterize this
system further by modeling the fluxes of DIN species and provide an
assessment of the ammonium oxidizing bacterial populations by using
molecular tools.
Publications:
Genin A., Lazar B. and Brenner S. (1995). Vertical mixing and coral deathin the Red
Sea following the eruption of Mounth Pinatubo. Nature. 377, 507-510.
Lindell D. and Post A.F. (1995). Ultraphytoplankton succession is triggered by deep
winter mixing in the Gulf of Aqaba (Eilat). Limnol. Oceanogr. 40(6), 1130-1141.
Moore L., Rocap G., Post A.F. and Chisholm S.W. (submitted). Differential nitrogen
utilization of the marine cyanobacteria, Prochlorococcus and Synechococcus.
Limnol. Oceanogr.
Lindell D. and Post A.F. (2001). Expression of the nitrogen regulatory gene ntcA as
an indicator of ammonium acquisition by marine unicellular cyanobacteria. Appl.
Environ. Microbiol. 67, 3340-3349.